Note: Descriptions are shown in the official language in which they were submitted.
CA 02357573 2001-09-21
Doc. No. 11-62 CA CIP Patent
HIGH EFFICIENCY ELECTRO-OPTIC MODULATOR WITH EQUALIZED FREQUENCY
RESPONSE
Field of the Invention
The invention relates generally to the field of optical modulation. In
particular, the invention
relates to methods and apparatus for high-efficiency electrc>-optic
modulation.
Back rg ound of the Invention
Optical modulators impress or modulate RF (or microwave) electrical signals
onto a light beam in
order to generate a modulated optical beam that carries data. Modulators
either directly modulate the
optical beam as it is generated at the optical source or externally modulate
the optical beam after it has
been generated. Direct modulation may be accomplished by modulating the drive
current of the optical
source. Direct modulation may also be accomplished by modulating the optical
intensity of light leaving
l:he source with an integrated electro-absorptive modulator.
External modulation can be accomplished by using an external modulator that is
separate from
the optical source. External modulation is advantageous because it can
modulate signals over a very wide
bandwidth. External modulators are typically voltage-controlled devices that
include a traveling-wave
c;lectrode structure, which is positioned in close proximity to the optical
waveguide. The electrode
structure produces an electric field that overlaps the optical waveguide over
a predetermined distance (the
interaction length) and causes an electromagnetic interaction, which modulates
the optical signal.
Lithium niobate (LN) electro-optic external modulators are increasingly being
used to modulate
data on optical signals that are being transmitted at very high data rates and
over long distances. Lithium
niobate modulators are advantageous because they can modulate optical signal
over a broad frequency
range, they modulate optical signals with controlled, potentially zero,
optical frequency shift (frequency
"'chirp"), and they operate over a broad wavelength range. 'these features are
particularly desirable for
Dense Wavelength Division Multiplexing (DWDM) broadband optical communication
systems that
transmit optical signals with many optical wavelengths through a single
optical fiber.
Lithium niobate crystals have an inherent mismatch between the velocity of
optical and electrical
signals propagating through the crystal, which lowers modulation efficiency.
The RF propagation index
is significantly higher than the optical refractive index of lithium niobate.
That is, the lithium niobate
crystal slows the RF signal relative to the optical signal so that it takes
the RF signal a longer period of
CA 02357573 2001-09-21
Doc. No. 11-62 CA CIP Patent
time to travel over the interaction distance. Thus, the RF signal becomes out-
of phase with or "walks ofF'
the accumulated modulation on the optical signal.
This "walk off' lowers the modulation efficiency. Modulators used for
transmission at high
speeds and over long distances must be efficient to minimize the use of
electronic amplifiers and digital
drivers. Electronic amplifiers and digital drivers are costly and occupy
valuable space in the transmission
link. In addition, electronic amplifiers and digital drivers may fail and
lower the quality of service and
require expensive maintenance in the field.
Fig. 1 illustrates a top view of a prior art electro-optic device 10 that
increases modulation
efficiency b;y compensating for the velocity mismatch between the optical and
electrical signals
laropagating through the device by using phase reversal sections that are co-
linear with the optical
waveguide. The device 10 includes an optical waveguide l2 and RF electrodes 14
that are positioned in
zero degree phase sections 16 and phase reversal sections 18.
The phase reversal sections 18 periodically flip the RF electrodes 14 to
either side of the optical
waveguide 12 to produce a 180 degree phase shift in the RF signal relative to
the accumulated modulation
on the optical signal. The RF electrodes 14 are positioned to alternate
between the zero degree phase
:sections 16 and the phase reversal sections 18. The length of the zero degree
phase shift sections 16 is
chosen so that the RF signal "walks ofF' the accumulated modulation on the
optical signal approximately
1180 degrees before it is flipped 180 degrees in the phase reversal sections
18.
The prior art electro-optic device 10 of Fig. 1 has relatively low modulation
efficiency per unit
length. This is because the phase of the RF signal is modified with co-linear
sections that are positioned
at intervals crf 180 degrees. When the difference in phase between the RF and
optical signals approaches
180 degrees, the incremental increase in modulation depth with incremental
change in electrode length
approaches ~;ero. Therefore, the total length of the device must be
significantly increased in order to
achieve the required modulation.
Fig. 2 illustrates a top view of a prior art electro-optic device 30 that
increases modulation
efficiency b~- compensating for the velocity mismatch between the optical and
electrical signals
propagating through the device :30 by using co-linear but intermittent
interaction sections. The device 30
includes an optical waveguide 32 and RF electrodes 34 that are positioned to
alternate between an
interaction region 36 and a non-interaction region 38 relative to the optical
waveguide 32.
The length of the interaction region 36 is chosen so that the RF signal "walks
off ' the modulation
on the optical signal by as much as 180 degrees of phase shift before it is
routed away from the optical
,7
CA 02357573 2001-09-21
Doc. No. 11-62 CA CIP Patent
waveguide .32 in a co-linear direction and into the non-interaction region 38.
The length of the non-
interaction region 38 is chosen so that the RF signal becomes phase matched
with the accumulated
modulation on the optical signal at the end of the non-interaction region 38.
The prior art modulator of
Fig. 2 has non-interaction regions of substantial length that re-align the
phase of the modulating signal,
but introduce RF loss, and occupy device length.
Increasing the length of a lithium n iobate electro-optic modulator increases
the size of the
package. Increasing the size of the modulator package and the power supplies
is highly undesirable
because the space on transmitter boards and in transmission huts is very
limited. Efficiency is also a
major consideration, as more powerful electronic drivers require more space on
the transmitter card.
.State-of the-art DWDM transmission equipment occupies a, significant amount
of space because the
equipment includes electronics for numerous channels. Most transmission huts
were designed for much
more modest communication systems and are not very spacious. Many transmission
huts cannot be
expanded for various reasons.
Another disadvantage of prior art electro-optic modulators in Fig. 1 and 2 is
that these modulators
are not suitable for modulating digital signals. This is because these
modulators have non-linear phase
characteristics as a function of frequency response. 'therefore, the digital
pulse shapes are not preserved.
In addition, the efficiency is concentrated in a set of narrow band regions,
which is suitable for a square
wave signal, but is unsuitable for digital signals having an arbitrary bit
sequence. Other prior art
modulators correct the uniformity of efficiency with frequency by using a
periodic, Barker-code, phase
reversal locations along the modulator length. However, these prior art
modulators still have non-linear
phase as a function of frequency.
Some prior-art electro-optic modulators uses a buffer layer to achieve
velocity matching as
described in connection with Fig. 3. These prior art devices have non-
optimized modulation efficiency
because they preserve significant modulation beyond the required bandwidth.
Some prior art electro-optic modulators use z-cut lithium niobate. Using z-cut
lithium niobate is
advantageous because z-cut lithium niobate inherently provides better overlap
between optical and RF
fields and thus, has an inherently high modulation efficiency as compared with
x-cut lithium niobate
e:lectro-optic modulators. Z-cut lithium niobate electro-optic modulators,
however, experience bias drift
effects. Conductive buffer layers and charge bleed-off layers are typically
used to mitigate these bias drift
effects. Including a conductive buffer layer and charge bleed-off layer adds
significantly to the
fabrication cost associated with z-cut electro-optic modulators. In addition,
prior art z-cut lithium niobate
:3
CA 02357573 2001-09-21
Doc. No. 11-62 CA CLP Patent
~electro-optic: modulator preserve significant modulation beyond the required
bandwidth and, therefore do
not have optimized efficiency.
_Summary 0l~the Invention
The present invention relates to high-efficiency electro-optic modulation and
to electro-optic
modulators for modulating digital signals with equalized frequency response. A
discovery of the present
iinvention is that modulation efficiency of electro-optic modulators can be
increased by discarding excess
modulation efficiency in regions inside and outside the bandwidth of the
digital spectrum. Another
discovery of the present invention is that the modulation efficiency and the
frequency response of an
c;lectro-optic modulator can be independently optimized by first choosing an
electrode geometry that
corresponds to a modulation efficiency in the digital signal spectrum.
Compensation networks are then
added at various points along the length of the modulating electrode in order
to modify the frequency
response of the modulator to discard excess modulation efficiency where it is
not needed, and add
efficiency where it is needed.. Optimum modulation efficiency and frequency
response are achieved by
iterating the selection of the electrode geometry and position and type of
compensation network.
Accordingly, the present invention features an electro-optic modulator for
modulating a digital
signal, such .as a Mach Zehnder interferometric modulator, comprising an
optical waveguide formed in an
e:lectro-optic material that propagates an optical signal along a first
direction of propagation. A buffer
layer may be formed on the optical waveguide to at least partially velocity
match the electrical
nnodulation signal to an optical signal.
An electrical waveguide is formed on the electro-optic material and positioned
generally co-linear
relative to the optical waveguide and in electromagnetic communication with
the optical waveguide. The
electrical waveguide propagates an electrical modulation signal in the first
direction of propagation. The
electrical waveguide may provide chirp modulation.
The electrical waveguide includes hot and ground electrical waveguides
positioned proximate to
the arms of the Mach Zehnder interferometric modulator. In one embodiment, the
electrical waveguide
comprises a c;o-planner strip electrode. A driver is coupled to the electrical
waveguide. In one
embodiment, the driver has a frequency response that is complementary to an
electro-optic frequency
rcaponse of tile modulator.
The geometry of the electrical waveguide is selected to achieve a modulation
efficiency at a
frequency in a bandwidth of a digital spectrum. At least one of the buffer
layer thickness, buffer layer
dielectric constant, and electrode geometry are selected to achieve a
modulation effuciency or to maximize
CA 02357573 2001-09-21
Doc. No. I 1-62 CA CIP Patent
the modulation efficiency at the frequency in the bandwidth of the digital
spectrum. In one embodiment,
the electrical waveguide geometry is selected to achieve the modulation
efficiency at substantially a mean
frequency in the bandwidth the digital spectrum.
In one embodiment, the electrical waveguides comprise dual-drive electrical
waveguides
including a first and second pair of hot and ground electrical waveguides
positioned proximate to a first
and second .arm of the Mach Zehnder interferometric modulator, respectively. A
driver is coupled to each
pair of hot a.nd ground electrical waveguides. In one embodiment, a frequency
response of each driver is
complemenl:ary to the electro-optic frequency response of the modulator.
A compensation network is electrically coupled to the electrical waveguide at
a junction. The
compensation network may comprise a plurality of compensation networks where a
respective one of the
plurality of compensation networks is electrically coupled to the electrical
waveguide at a respective one
of a plurality of junctions. The compensation network modifies the electro-
optic response of the electro-
optic modulator below a mean frequency of the digital spectrum, thereby
causing a magnitude of the
electro-optic; response to increase in the bandwidth of the digital spectrum.
'the compensation network
imodifies at lleast one of a phase or an amplitude of the electrical
modulation signal at the junction relative
to a phase or an amplitude of the optical signal at the junction,
respectively.
There are numerous embodiments of the compensation network. The compensation
network may
iinclude an R.F time delay network or a polarity reversal section. In one
embodiment, the compensation
network propagates the electrical signal in a second direction of propagation
that is substantially non-co-
linear with the first direction of propagation. The electrical loss per unit
length of the compensation
network may be lower than an electrical loss per unit length of the electrical
waveguide. The
compensation network may be removably attached to the electro-optic modulator.
Also, the temperature
dependence of the compensation network may be inversely proportional to a
temperature dependence of
t:he electro-optic material.
The present invention also features a method of independently controlling
modulation efficiency
and electro-optic response of an electro-optic modulator for modulating a
digital signal. The method
includes self;cting a modulator length corresponding to a modulator bandwidth.
The modulation
efficiency is then adjusted at a frequency in the bandwidth of a digital
spectrum by selecting an electrical
waveguide geometry of the electro-optic modulator.
The magnitude of the electro-optic response is then reduced below a mean
frequency of the
digital spectrum. In one embodiment, the magnitude of the electro-optic
response is reduced electro-
CA 02357573 2001-09-21
Doc. No. 11-62 CA CIP Patent
optically. In some embodiments, the electro-optic response above the mean
frequency of the digital
spectrum is increased. The steps of choosing the modulator length, adjusting
the modulation efficiency,
.and reducing the magnitude of the electro-optic response below the mean
frequency of the digital
spectrum m~ty by iterated to achieve a predetermined modulation efficiency and
electro-optic response in
the bandwidth of the digital spectrum.
The present invention also features a method of independently optimizing the
modulation
efficiency and electro-optic response of an electro-optic modulator for
modulating a digital signal. The
method includes selecting a modulator length that corresponds to a modulator
bandwidth. The
modulation efficiency is then optimized at a frequency in the bandwidth of a
digital spectrum by selecting
an electrical waveguide geometry of the electro-optic modulator.
The electro-optic response is optimized above the mean frequency of the
digital spectrum by
reducing a magnitude of an electro-optic response below the mean frequency of
the digital spectrum. The
<.~teps of selecting the modulator length, optimizing the modulation
efficiency, and optimizing electro-
optic response above the mean frequency of the digital spectrum may by
iterated to achieve a
predetermined modulation efficiency and electro-optic response in the
bandwidth of the digital spectrum.
H3rief Descri ption of the Drawings
This invention is described with particularity in the appended claims. The
above and further
advantages of this invention may be better understood by referring to the
following description in
conjunction with the accompanying drawings, in which like numerals indicate
like structural elements and
features in v;~rious figures. The drawings are not necessarily to scale,
emphasis instead being placed upon
illustrating the principles of the invention.
Fig. 1 illustrates a top view of a prior art electro-optic device that
increases modulation efficiency
by compensating for the velocity mismatch between the optical and electrical
signals propagating through
tike device by using phase reversal sections that are co-linear with the
optical waveguide.
Fig. 2 illustrates a top view of a prior art electro-optic device that
increases modulation efficiency
b~y compensating for the velocity mismatch between the optical and electrical
signals propagating through
the device by using co-linear but intermittent interaction sections.
Fig. :3 illustrates an electrode cross-section of a prior art traveling wave
electro-optical modulator
v~rhere the optical and electrical signals propagate in an entirely co-linear
path.
6
CA 02357573 2001-09-21
Doc. No. 11-62 CA CIP Patent
Fig. 4 illustrates a top view of an equalized Mach-Zehnder x-cut single-drive
electro-optic
modulator according to the present invention.
Fig. 5 illustrates a top view of one embodiment of a compensation network for
the equalized
:Mach-Zehnder x-cut electro-optic modulator of Fig. 4 that includes an RF
delay line, and polarity reversal
due to the lateral electrode shift.
Fig. 6 illustrates a top view of one embodiment of a compensation network and
equalization
section for the equalized Mach-Zehnder x-cut electro-optic modulator of Fig. 4
that includes an RF delay
line and a polarity reversed equalization section.
Fig. 7 illustrates a graph of calculated data for electro-optic phase as a
function of frequency for
the equalized Mach-Zehnder x-cut single drive electro-optic modulator of the
present invention.
Fig. 8 illustrates a graph of calculated data for modulation efficiency as a
function of frequency .
response comparing conventional x-cut modulators and the equalized Mach-
Zehnder x-cut single drive
e;lectro-optic modulator of the present invention.
Fig. 9 illustrates a top view of an equalized Mach-Zehnder z-cut single-drive
electro-optic
modulator according to the present invention.
Fig. 10 illustrates a top view of one embodiment of a compensation network for
a z-cut modulator
according to the present invention that includes an RF delay line and a
polarity reversal section.
Fig. 11 illustrates a top view of an equalized Mach-Zehnder x-cut dual-drive
electro-optic
nnodulator according to the present invention.
Fig. l2a,b illustrates top views of two embodiments of compensation networks
for the equalized
Mach-Zehnder x-cut dual-drive electro-optic modulator of Fig. 11.
Detailed Descri tion
Fig. 3 illustrates an electrode cross-section of a prior art traveling wave
electro-optical modulator
50 where the optical and electrical signals propagate in an entirely co-linear
path. The substrate 52 is
firmed of an electro-optic material, such as lithium niobate (LiNb03). Many
prior art modulators use X-
cut LiNb03 substrates. The modulator includes optical waveguides 54 that are
positioned entirely within
the substrate 52. The optical waveguides 54 may be formed by diffusing
titanium into the LiNb03
substrate 52. Electrical waveguides are positioned in a co-planar waveguide
(CPW) geometry with a hot
electrode 56 positioned in the center of the CPW structure and ground
electrodes 58 positioned adjacent
'7
CA 02357573 2001-09-21
Doc. No. 1 I-62 CA CIP Patent
to both sides of the hot electrode 56. The ground electrodes 58 are spaced
apart from the hot electrode by
a gap 60. Typical prior art modulator lengths are on order of 20 to 50 mm.
Optical and electrical modulation signals propagate along the optical 54 and
electrical
waveguides 56, 58, respectively, into the plane of the page. The electrical
modulation signal generates an
electric field 62 that travels from the hot electrode 56 to the ground
electrodes 58.. It is desirable to
minimize the required amplitude of the electrical modulation signal. The gap
60 is typically chosen to be
;small in order to maximize the electromagnetic interaction of the electrical
waveguides 56, 58 with the
optical wavt~guides 54 and thus minimize the amplitude of the required
electrical modulation signal.
The required amplitude of the electrical modulation signal is determined by
the switching
voltage-length product (Vpi-L) of the modulator, which is a function of both
the electrode geometry and
the degree of overlap between the electric field 62 and the optical signal.
Vpi-L of typically prior art
modulators is typically 40 to 50 Volt-mm for the CPW structure shown in Fig.
3. Thus for a modulator
with 20 mm long electrodes, the required electrical modulation signal is
approximately 2 volts.
A disadvantage of the prior art modulator 50 is that at high modulation
frequencies, RF losses
become significant. Typical RF losses range from about 0.5 to 1 dB/Sqrt(GHz)-
cm for LiNbO~.
Consequently, the electromagnetic interaction of the electrical waveguides 56,
58 with the optical
waveguides 54 decreases at high frequency thereby increasing the Vpi-L and
thus, the required amplitude
of the electrical modulation signal.
Another disadvantage of prior art modulators is that the velocities of the
optical signal and the
electrical modulation signal are different. The velocity of the electrical
modulation signal is determined
by the RF propagation index, which may be as high as 3.5 to 4Ø The velocity
ofthe optical signal is
determined by the optical propagation index, which is approximately 2.1 for
LiNb03. The difference in
t:he RF index; and the optical propagation index results in a significant
velocity mismatch of the optical
and the electrical modulation signals. This velocity mismatch results in the
modulation on the optical
signal becoming out-of phase with the electrical modulation signal, thereby
reducing or canceling the
accumulated modulation, and increasing the amplitude of the required
electrical modulation signal and
lower the modulation efficiency.
Somae prior art modulators include a buffer layer 64 that is positioned
behveen the electrodes 56,
:i8 and the substrate 52 that has a lower dielectric constant than the
substrate 52. using the buffer layer
ti4 lowers the RF index because the buffer layer 64 together with the
substrate 52 presents an average
f;ffective RF index that is somewhere between the RF refractive index of the
buffer layer 64 and the RF
8
CA 02357573 2001-09-21
Doc. No. 11-62 CA CIP Patent
index of the substrate. Using a buffer layer, however, degrades the efficiency
of the modulator because
the electric :field must penetrate a longer path and thus is reduced in
magnitude.
Modulator efficiency and frequency response are strongly coupled parameters in
these prior-art
lithium niob~ate broadband modulators. ')-'he frequency response of these
modulators typically drops off in
.an approximate linear function with increasing frequency. Adding incremental
efficiency at high
frequencies, near the mean of a digital signal spectrum, typically also adds
efficiency at lower
frequencies, but at an even faster rate than near the mean of the digital
signal spectrum. Consequently,
optimizing the electrode structure for maximum efficiency at the mean of the
digital signal spectrum
usually results in excessive efficiency at low frequencies, and therefore
excessive roll-off in modulation
response with frequency. Thus, even though these prior art modulator are more
efficient at high
frequencies, they will have slow rise and fall times in response to a digital
signal because of an inadequate
:3dB bandwidth.
Elec;tro-optic modulators of the present invention use compensation networks
to electro-optically
discard electro-optic efficiency inside or outside of the bandwidth of the
digital spectrum. In one
embodiment:, compensation networks are used to create a frequency response
that is substantially flat up
to some desired corner frequency, which is near the bandwidth of the
modulator. The frequency response
then drops off rapidly above the corner frequency. 'therefore, the modulation
response at very high
frequencies is discarded. This is desirable because modulation components at
very high frequencies can
lie corrupted by fiber dispersion in a communication link and they provide no
system performance
benefit.
Fig. 4 illustrates a top view of an equalized Mach-Zehnder x-cut single-drive
electro-optic
modulator 100 according to the present invention. The modulator 100 includes
an optical input 102, a
t~rst 104 and second optical waveguide 104' forming two arms of a Mach-Zehnder
interferometer, and an
optical output 106. The optical waveguides 104, 104' are fbrmed in an x-cut
lithium niobate substrate as
described in connection with Fig. 3 or by numerous other techniques known in
the art.
A hot electrical waveguide 107 and a ground electrical waveguide 108 are
positioned in a co-
planar strip electrode geometry along one arm of the interferometer. A guard
ground electrode 109 may
be positioned adjacent to the optical waveguides 104, 104' to achieve thermal
balancing. The electrical
waveguides 107, 108 propagate electrical modulation signals proximate to the
optical waveguides 104,
104'. In one embodiment, the electrical modulation signals are partially
velocity matched to the optical
signals. In this embodiment, a thin buffer layer is used to achieve partial
velocity matching. In one
embodiment, the electrodes provide chirped modulation, which is useful for
some applications.
9
CA 02357573 2001-09-21
Doc. No. 11-62 CA CIP Patent
Nurn, erous other waveguide geometries can be used. Coplanar strip electrodes
are advantageous
because they reduce bias point drift compared to prior art asymmetric coplanar
waveguides. In addition,
coplanar strip electrodes are advantageous because it is relatively easy to
construct RF delay lines and
polarity reve;rsal sections using coplanar strip electrodes. Also, it is
relatively easy to mate the modulator
t:o a various output drivers including complementary output drivers because
the electrode structure does
not have to be modified.
The modulator 100 includes a main modulation section 110 and an equalization
section 112. A
compensation network 114 is electrically coupled between the main modulation
section 110 and the
equalization section I 12. The compensation network 1 14 may be integrated
into the hot electrical
waveguide 107 and the ground electrical waveguide 108 or may be a separate
device that is electrically
connected, but removably attached to the modulator 100. A removably attached
compensation network is
advantageous because it can be easily reconfigured.
The compensation network 114 is used to equalize the electro-optic response.
By electro-optic
response, we mean the ratio of the magnitude and phase of an RF output voltage
generated by a
photodetector that detects the modulated optical signal propagating from the
optical output 106 to the
magnitude and phase of the RF modulation signal. The electro-optic response
represents the accumulated
modulation on the optical signal for either section of the modulator, or the
entire modulator.
The compensation network 114 modifies at least one of the phase or the
amplitude of the
electrical signal relative to the phase or amplitude ofthe accumulated
modulation on the optical signal. In
one embodiment, the compensation network I 14 is designed to achieve a
substantially flat electro-optic
response in phase and magnitude from DC to a cut-off frequency. Above the cut-
off frequency, the
electro-optic response attenuates rapidly.
One .advantage of the present invention is that the compensation is performed
electro-optically
rather than purely electrically. Electro-optic compensation is physically more
compact than prior art
electrical compensation methods because the compensation structure is
integrated in the device. Electro-
optic compensation is also more efficient than prior art electrical
compensation because the electro-optic
compensation of the present invention has much less electrical insertion loss.
The insertion loss is less
because the compensation networks are physically integrated into the device
and, therefore, RF coupling
losses are minimal because the modulator and the compensation network have
similar electrode
structures. Also, the insertion loss is less because the compensation networks
of the present invention are
typically located downstream of the RF input and, therefore, the RF performs
some modulation before the
compensation network.
CA 02357573 2001-09-21
lDoc. No. I I-ii2 CA CIP Patent
In one embodiment, the compensation network 114 temporarily directs the
electrical signal in a
path that is in a non-co-linear direction relative to the direction of
propagation of the optical signal. The
compensation network may comprise an Rf delay line and polarity reversal as
described in connection
with Fig. 5 that modifies the frequency response. The compensation network may
also comprise an RF
delay line and a polarity reversal section as described in connection with
Fig. 6 that modifies the
iFrequency reaponse. For example, the frequency response of the compensation
network I 14 may be
modified so that it is substantially flat to a corner frequency and then
rapidly attenuates at frequencies
above the corner frequency.
In one embodiment of the present invention, the compensation network 114 is
designed to
I;enerate frequency response characteristics that vary with at least one
external perturbation in a manor
that compensates for such perturbation. For example, the compensation network
l 14 may be designed to
I;enerate frequency response characteristics that vary with temperature in
such a way as to compensate for
the effect of temperature variations in the electro-optic substrate. This can
be accomplished by varying
the physical length, dielectric constant, or other parameters of the
compensation network. In one
c;mbodiment, a plurality of compensation networks I 14 is positioned
periodically along the modulator
100. In other embodiments, a plurality of compensation networks 114 is
positioned non-periodically
along the modulator. There are numerous advantages to the compensation network
of the present
invention. C>ne advantage is that such a compensation network can modify the
phase of the electrical
signal relative to the optical signal in order to minimize the effects of
velocity mismatch, while
introducing very low loss. The compensation network 114 can be designed so
that the electrical loss per
unit length is significantly lower than the electrical loss per unit length of
the electrical waveguide. For
example, the electrodes comprising the compensation network 114 can be
constructed with wider
conductors that have relatively low resistive losses a.nd wider gaps between
the conductors, which reduce
skin effectlosses.
Fig. 5 illustrates a top view of one embodiment of a compensation network 1 SO
for the equalized
rvlach-Zehnder x-cut electro-optic modulator of Fig. 4 that includes an RF
delay line 152, and polarity
reversal due to the lateral electrode shift The compensation network 150
comprises a hot electrical
waveguide 154 and ground electrical waveguide I 56 that temporarily directs
the electrical signal to the
F~F delay line: section 152 that is in a non-co-linear direction relative to
the direction of propagation of the
optical signal. The RF delay line section 152 has a length that causes a
predetermined RF delay relative
to the accumulated modulation that is variable over a range from zero to one
hundred and eighty degrees.
In one embodiment, this predetermined delay is substantially one hundred and
ten degrees.
11
CA 02357573 2001-09-21
Doc. No. 11-~52 CA CIP Patent
The compensation network 150 modifies at least one of the phase and the
amplitude of the
electrical signal relative to the phase and amplitude of the accumulated
modulation on the optical signal.
1(n one embodiment, compensation network 150 modifies only the phase and the
amplitude is substantially
unchanged. Fringing fields from the hot electrical waveguide I 54 and ground
electrical waveguide 156
cause some :modulation of the adjacent waveguide, which changes polarity after
the compensation
network. In one embodiment, the frequency response of the compensation network
150 is substantially
slat to a corner frequency and then rapidly attenuates at frequencies above
the corner frequency.
In another embodiment, the frequency response of the compensation network 150
is substantially
flat with no corner frequency up to the frequency where the response drops off
because of velocity walk-
off. In this embodiment, the compensation network aligns the phases of the RF
and accumulated
modulation over a narrow band of frequencies near the high end of the
operating band, thereby causing
some peaking of frequency response. The peaking in the frequency response near
the high end
compensates for the natural roll-off of the modulator and, therefore, produces
a flatter frequency response
in the bandwidth of the digital spectrum. The compensation network may include
a polarity reversal
section as described in connection with Fig. 5, Fig. 6 and Fig. 10 that causes
a reduction in the frequency
response at the low end of the operating band.
The equalized Mach-Zehnder electro-optic modulator 100 of Fig. 4 using the
compensation
network 150 of Fig. 5 operates in the following way. An optical signal is
coupled to the optical signal
input 102 of the modulator 100. An electrical modulation signal is coupled to
the hot electrical
vraveguide 1 X07 and the ground electrical waveguide I 08. The optical signal
propagates along the optical
vraveguides I 04, 104' in the main section of the modulator l 00. The
electrical modulation signal
propagates al',ong the hot electrical waveguide 107 and the ground electrical
waveguide 108 until it enters
the compensation network 150.
When the electrical modulation signal enters the RF delay line section 152,
the electrical
modulation signal does not significantly electromagnetically interact with the
optical signal. The RF
delay line section 152 modifies the phase of the electrical modulation signal
relative to the phase of the
accumulated modulation on the optical signal, and then returns the compensated
modulation signal to the
equalization ;section 112 of the modulator 100.
Fig. ti illustrates a top view of one embodiment of a compensation network and
polarity-reversed
equalization section 160 for the equalized Mach-Zehnder x-cut electro-optic
modulator of Fig. 4 that
includes an RF delay line and a polarity reversal equalization section 164..
The RF delay line section 162
comprises a hoot electrical waveguide 166 and a ground electrical waveguide
168 that temporarily directs
12
CA 02357573 2001-09-21
Doc. No. 11-52 CA CIP Patent
the electrical signal to the RF delay line section 162 that is in a non-co-
linear direction relative to the
direction of propagation of the optical signal.
The RF delay line section 162 has a length that causes a predetermined RF
delay. The
predetermined RF delay is variable over a range from zero to one hundred and
eighty degrees. The
polarity reversal section 164 comprises a hot electrical waveguide 166' and a
ground electrical waveguide
168' that are: positioned in a co-linear, but opposite direction relative to
the direction of propagation of the
optical signal. Polarity reversal is thus achieved by flipping the orientation
of the signal relative to the
optical wave~guide. The compensation network and equalization section of Fig.
6 is advantageous in that
the equalization section and main modulation section modulate the same optical
waveguide. The
disadvantage; is that less electro-optic response will 'be available from the
equalization section at high
frequency due to increased velocity mismatch from counter propagation.
Fig. 7 illustrates a graph of calculated data for electro-optic phase as a
function of frequency for
the equalized Mach-Zehnder x-cut single drive electro-optic modulator 100 of
the present invention using
the compensation network of Fig. 5. The electro-optic phase is flat to within
about +/- 7 degrees up to
approximately 13 GHz
Fig. 8 illustrates a graph of calculated data for modulation efficiency in -
Ppi in dbm as a function
of frequency in GHz comparing a conventional x-cut modulator and the equalized
Mach-Zehnder x-cut
single drive c~lectro-optic modulator 100 of the present invention. Three
modulation efficiency frequency
response curves are illustrated. The first curve 170 illustrates the
modulation efficiency as a function of
frequency response for a conventional 1 OGb x-cut modulator.
The second curve 172 illustrates modulation efficiency as a function of
frequency response for an
optimized 1 OGb x-cut modulator without using the electro-optic equalization
of the present invention.
The optimized modulator includes electrical waveguides positioned in a co-
planar strip electrode
geometry and a relatively thin buffer layer that partially velocity matches
the electrical modulation signal
applied to thc: electrical waveguide and the optical signal applied to the
optical waveguide of the electro-
optic modulator. The third curve 174 illustrates modulation efficiency as a
function of frequency
response for an optimized 1 OGb x-cut modu lator using the electro-optic
equalization of the present
invention using the compensation network of Fig. 5.
T'he equalized Mach-Zehnder x-cut single drive electro-optic modulator 100 of
the present invention
exhibits swite:hing voltages for a l OGb/s data rate that are comparable to
state-of the art conventional z-
cut modulators, but with x-cut fabrication technology. For example, the
switching voltage for a SOS
13
CA 02357573 2001-09-21
lDoc. No. I 1-G2 CA CIP Patent
driver may E>e less than SV, if an impedance transformer is used between the
driver and the modulator.
Thus the modulator 100 provides an efficiency improvement that does not
require a costly conductive
buffer and charge bleed layer. In addition, the equalized Mach-Zehnder x-cut
single drive electro-optic
modulator 1 ~00 of the present invention more effectively extinguishes the
modulation response above the
required bandwidth compared with prior art x-cut devices.
The PRBS extinction after 80 km of SMF is calculated to be approximately 11
dB, which is
comparable to optimized z-cut devices. This PRBS is approximately 2 dB higher
than conventional x-cut
devices because the chirp parameter is -0.5. Note that fiber non-linearities,
receiver noise, and clock
recovery were not included in these calculations.
One advantage of the equalized Mach Zehnder modulator of the present invention
is that the
equalization provided by the compensation networks allows designers to design
electrodes that are
optimized independently of bandwidth. Thus, with the equalized Mach Zehnder
modulator of the present
invention, the modulation efficiency and the electro-optic response can be
independently controlled.
For example, one method of independently controlling the modulation efficiency
and the electro-
optic response includes selecting a modulator length that corresponds to a
modulator bandwidth. The
modulation efficiency is then adjusted at a frequency in the bandwidth of a
digital spectrum by selecting
an electrical waveguide geometry of the electro-optic modulator.
The waveguide geometry may be selected by choosing at least one of the buffer
layer thickness,
buffer layer dielectric constant, and electrical waveguide geometry to
partially velocity match the
electrical modulation signal applied to the electrical waveguide and the
optical signal applied to the
optical waveguide of the electro-optic modulator. In one embodiment, the
modulation efficiency is
adjusted at substantially a mean frequency of the bandwidth of the digital
spectrum.
Meats frequency of the bandwidth of the digital spectrum is herein defined to
be the frequency at
which half the total integrated power is below the mean and half the total
integrated power is above the
mean. The v~~ltage required to drive a modulator at the mean frequency of the
digital spectrum provides
an estimate of the switching voltage for the modulator, in response to a
digital signal. The response at the
mean frequency is a useful tool for designing modulators because calculation
or measurement of electro-
optic response at one frequency is typically easier than applying a digital
signal (either in theory or
experiment).
14
CA 02357573 2001-09-21
lDoc. No. 11-ii2 CA CIP Patent
The magnitude of the electro-optic response is then reduced below the mean
frequency of the
digital spectrum thereby increasing an electro-optic response above the mean
frequency of the digital
spectrum.
In one embodiment, the magnitude of the electro-optic response in the
bandwidth of the digital
spectrum is modified by electrically coupling at least one compensation
network to a junction of the
electrical wa.veguide. For some applications a plurality of compensation
networks are coupled to a
plurality of junctions of the electrical waveguide. In one embodiment, the
compensation networks time
delay the electrical modulation signal and/or reverses the phase of the
electrical modulation signal. The
rnethod of choosing the modulator length, adjusting the modulation efficiency,
and adjusting the
magnitude of the electro-optic response may by repeated or iterated to achieve
a predetermined
modulation efficiency and electro-optic response in the bandwidth of the
digital spectrum.
Also, with the equalized Mach Zehnder modulator of the present invention, the
modulation
efficiency and the electro-optic response can be independently optimized. One
method of independently
optimizing the modulation efficiency and the electro-optic response includes
selecting a modulator length
tlhat corresponds to a modulator bandwidth. The modulation efficiency is then
optimized at a frequency
in the bandwidth of a digital spectrum by selecting an electrical waveguide
geometry of the electro-optic
modulator. 'hhe modulation efficiency may be optimized at substantially a mean
frequency of the digital
spectrum.
The electro-optic response is optimized above the mean frequency of the
digital spectrum by
reducing a magnitude of an electro-optic response below the mean frequency of
the digital spectrum. In
one embodiment, the magnitude of the electro-optic response in the bandwidth
of the digital spectrum is
optimized by electrically coupling a compensation network to a junction of the
electrical waveguide. The
steps of selecting the modulator length, optimizing the modulation efficiency,
and optimizing electro-
optic response above the mean frequency of the digital spectrum may by
iterated to achieve a
predetermined modulation efficiency and electro-optic response in the
bandwidth of the digital spectrum.
The modulator of the present invention can also be constructed from z-cut
lithium niobate. Fig. 9
illustrates a top view of an equalized Mach-Zehnder z-cut single-drive electro-
optic modulator 190
according to the present invention. The modulator 190 is similar to the
modulator 100 of Fig. 4. The
modulator 190 includes an optical input 102, a first 104 and second optical
waveguide 104' positioned in
a Mach-Zehnder configuration, and an optical output 106. The optical
waveguides 104, 104' are formed
in a z-cut lithium niobate substrate.
1~
CA 02357573 2001-09-21
lDoc. No. 11-62 CA CIP Patent
A hot electrical waveguide 107 and a ground electrical waveguide 108 are
positioned in a co-
planar strip electrode geometry that propagates the electrical modulation
signal along the electrodes.
Numerous other waveguide geometries can be used. In one embodiment, the
electrodes provide chirped
modulation, which is useful for some applications. 'fhe modulator 190 includes
a main modulation
section 110 ;end an equalization section I 12.
A compensation network 114 is electrically coupled between the main modulation
section 110
and the equalization section 112. The compensation network 114 may be
integrated into the hot electrical
waveguide 107 and the ground electrical waveguide l08 or may be a separate
device that is electrically
connected, but removably attached to the modulator 100. T'he modulator 190 is
compatible with push-
pull, complementary output drivers if appropriate impedance transformations
are used between the driver
amd the modulator.
Fig. 10 illustrates a top view of one embodiment of the compensation network
192 for a z-cut
nnodulator according to the present invention that includes an RF delay line
section 194 and a polarity
reversal section 196. The compensation network 192 is similar to the
compensation network 150 that was
described in connection with Fig. 5, but is positioned for a z-cut modulator.
In addition, the
compensation network 192 includes a bridge 198 for one electrode that provides
polarity reversal.
Fig. 11 illustrates a top view of an equalized Mach-Zehnder x-cut dual-drive
electro-optic
modulator 200 according to the present invention. The dual-drive electro-optic
modulator 200 is similar
to the x-cut single-drive electro-optic modulator 100 of Fig. 4, except the
modulator 200 has modulation
electrodes positioned around both arms the Mach Zhender interferometer. The
modulator 200 includes an
optical input 102, a first 104 and second optical waveguide 104' forming two
arms of a Mach-Zehnder
interferometer, and an optical output 106.
A pair of hot electrical waveguides 107 and ground electrical waveguides 108
is positioned in a
co-planar strip electrode geometry around each arm of the interferometer. The
electrical waveguides 107,
108 propagate electrical modulation signals proximate to the optical
waveguides 11)4, 104'. In one
embodiment, the electrical waveguides 107, 108 are partially velocity matched.
In this embodiment, a
thin buffer layer is used to achieve partial velocity matching. In one
embodiment, the electrical
waveguides 107, 108 provide chirped modulation, which is useful for some
applications.
The dual-drive electro-optic modulator 200 includes main modulation sections
110 and
equalization sections 112. Compensation networks 114 are electrically coupled
between the main
modulation section 1 10 and the equalization section 112 of each pair of
waveguides 107, 108 to equalize
16
CA 02357573 2001-09-21
Doc. No. 11-ei2 CA CIP Patent
t:he electro-optic response as described in connection with Fig. 4. The
compensation networks 114 may
be integrated into the waveguides 107, 108 or may be separate devices that is
electrically connected, but
removably attached to the modulator 100.
The dual-drive electro-optic modulator 200 is advantageous because such a
modulator can
produce an overall efficiency that is flat over the required bandwidth and
high enough for use with a flat-
pack complementary driver having outputs 202 and 202'. In one embodiment, the
drivers are provided by
two outputs from the same driver. The drivers may include an RF launching and
impedance matching
structure that is attached to the electrical waveguides 107, 108. By adjusting
the modulator
impulse/frequency response to be complementary with the driver, the overall
impulse/frequency response
of the electro-optic modulator including the drivers can be made optimal.
Fig. 12a illustrates a top view of one embodiment of a compensation network
220 for the
equalized Mach-Zehnder x-cut dual-drive electro-optic modulator 200 of Fig. 1
1. The compensation
networks 220 is similar to the compensation network of Fig. 6, except that
each arm of the Mach-Zehnder
interferometer includes a compensation network 22G. Fig. 12b illustrates a top
view of another
embodiment of a compensation network 230 for the equalized Mach-Zehnder x-cut
dual-drive electro-
optic modulator 200 of Fig. I 1. The compensation network 230 is similar to
the compensation network of
E~ ig. 10.
Equivalents
While the invention has been particularly shown and described with reference
to specific
preferred embodiments, it should be understood by those skilled in the art
that various changes in form
a.nd detail may be made therein without departing from the spirit and scope of
the invention as defined by
tlhe appended claims. For example, the present invention applies to z-cut
lithium niobate electro-optic
modulators as well as x-cut lithium niobate electro-optic modulators.
17
